1. Field of the Invention
The present invention generally relates to a driving control method and circuit for electromagnetic servo devices. More particularly, the invention relates to a driving control method and circuit for an electromagnetic servo device adapted to be employed in an electric type power steering system for vehicles.
2. Description of Relevant Art
In view of problems on the hydraulic type power steering system such as that the structure thereof was complicated, recent years have proposed a variety of electric type power steering systems for vehicles.
In those electric type power steering systems were employed various types of electromagnetic servo device.
Those types of electromagnetic servo device included an input shaft adapted to be connected to a steering wheel, an output shaft adapted to be connected, through a steering gearbox or otherwise, to a tie rod of a road wheel to be steered, a torque detection mechanism for detecting the magnitude as well as the direction of torque developed at the input shaft due to a steering force acting on the steering wheel in relation to a load at the end of the tie rod, an electric motor for supplying the output shaft with auxiliary torque, and a driving control circuit for sending to the electric motor an electric current of such a magnitude and in such a direction as necessary in accordance with a detection signal from the torque detection mechanism.
As an example thereof, there has been disclosed an electromagnetic servo device in Japanese Patent Lay-Open Print No. 59-70257, laid open on Apr. 20, 1984.
In this electromagnetic servo device, as a torque detection mechanism, a strain gauge sensor was disposed on an input shaft and, on the basis of an output signal from the sensor, there were generated a torque direction signal representing the direction of torque acting on the input shaft and a torque magnitude signal representing the magnitude of the torque in terms of an absolute value, to send to an electric motor an electric current of such a magnitude and in such a direction as necessary in accordance with the respective signals, so that an output shaft was supplied with necessary auxiliary torque.
Incidentally, as shown in FIG. 7A, in those electromagnetic servo devices in which, as shown in FIG. 7A, a circuit for generating a torque magnitude signal Sa had an inherent dead zone DZ in a torque region where the magnitude of torque acting on an input shaft was in the vicinity of zero, there conventionally was a problem such that an electric motor was unable to start when a steering wheel was rotated with a small steering force.
In view of this problem, in the electromagnetic servo device according to the aforesaid Japanese Patent Lay-Open Print, as shown in FIG. 7B, the torque magnitude signal, Sa, was biassed by a voltage .DELTA.V in only such a region that constituted a dead zone DZ in the generation thereof. As a result, the electric motor was always permitted to be controlled in its driving with an electric current of adequate magnitude even while the torque acting on the input shaft was small.
However, as will be detailed later with reference to FIGS. 8 and 9, in this electromagnetic servo device, in which the torque direction signal (Sd) was generated in accordance with a torque detection signal substantially of the same form as the torque magnitude signal (Sa) shown in FIG. 7A, in a torque region where the signal state of the direction signal (Sd) had to be changed from "on" to "off", that is, from "high" to "low" or vice versa, the electric motor as to become fed with no electric current just at the time when the state of the signal (Sd) was changed from "on" to "off" and become fed with an electric current of a certain magnitude just when this signal state was changed from "off" to "on", so that a hunting was likely to occur in such torque region. In other words, in this servo device, in a torque region where the torque acting on the input shaft was small, the state of the torque direction signal (Sd) was to be interchanged between "on" and "off" from time to time, resulting in the tendency to cause hunting.
FIG. 8 is a graph showing a relation between a torque direction signal Sd and a torque magnitude signal Sa in a conventional type of electromagnetic servo device covering the electromagnetic servo device according to the aforesaid Japanese Patent Lay-Open Print.
In this conventional type of electromagnetic servo device, for generating the torque direction signal Sd depending on a detection signal from a torque detection mechanism, there was employed a voltage comparison circuit such as a Schmidt trigger circuit, which generally has a hysteresis characteristic.
In FIG. 8, the axis of abscissa represents the magnitude and direction of torque acting on an input shaft, the abscissa corresponding at the right side of the origin O to clockwise rotation of a steering wheel and at the left side of same to counterclockwise rotation of the steering wheel, and the axis of ordinate represents the value of respective voltages defining the torque magnitude and direction signals Sa, Sd.
As shown in FIG. 8, when plotted, the torque magnitude signal Sa, the voltage of which was correspondent to the magnitude of the input torque and by which the absolute value of the armature current of an electric motor was controlled to be dependent thereon, gave a valley-like characteristic curve having at the bottom thereof a dead zone DZa, and the torque direction signal Sd, which consisted of a pair of signals Sd.sub.1, Sd.sub.2 responsible either at Sd.sub.1 for the clockwise rotation of the steering wheel and the other at Sd.sub.2 for the counterclockwise rotation of same and depending on which the conduction of the armature current of the electric motor was controlled in the direction (polarity) thereof to be in accordance with the rotational direction of the input torque, gave a pair of stepped characteristic curves representing the clockwise rotation signal Sd.sub.1 and the counterclockwise rotation signal Sd.sub.2, respectively, the stepped curves cooperating with each other to define a dead zone DZd therebetween, while having shown at the phase of stepping such a hysterestic nature as represented by right and left hysteresis loops H.sub.1 and H.sub.2. The respective signals Sa and Sd (Sd.sub.1, Sd.sub.2) were fed to a driving control circuit of the electric motor.
In this conventional type of electromagnetic servo device, in which actually the torque direction signal Sd consisting of the signals Sd.sub.1 and Sd.sub.2 was generated at the aforementioned voltage comparison circuit on the basis of the torque magnitude signal Sa, the dead zone DZd in the generation of the direction signal Sd was set wider than the dead zone DZa in that of the magnitude signal Sa.
Incidentally, in FIG. 8, the signals Sd.sub.1, Sd.sub.2 have minimum values thereof shown as though they had been above zero for the convenience of distinction thereof, whereas these minimum values were all substantially zero.
FIG. 9 is a graph showing, for various magnitudes in both rotational directions of the input torque, the armature current of the electric motor, Am, as it was when the signals Sa, Sd.sub.1, Sd.sub.2 were varied as shown in FIG. 8. In FIG. 9 also, the armature current Am has a minimum value thereof shown as if it had been apparently above zero for easier comprehension, whereas this value was close to zero.
As shown in FIG. 9, the armature current Am had a hysterestic nature that was represented by right and left hysteresis loops H.sub.3, H.sub.4 due to the right and left hysteresis loops H.sub.1, H.sub.2 of the rotational direction signals Sd.sub.1, Sd.sub.2, respectively, as well as a dead zone corresponding to the dead zones DZa, DZd of the torque magnitude and direction signals Sa, Sd. Due to presence of the dead zone, in a region where the torque acting on the input shaft was within a small magnitude, the armature current Am had a "high" level range and a "low" level state inconsistent from each other though very close to each other. The hysterestic nature was such that the magnitude of the armature current Am experienced in each direction on the conduction thereof a sudden rise from the "low" state to a "high" level A.sub.1 at a value of magnitude T.sub.1 (for clockwise direction) or T.sub.2 (for counterclockwise direction) of the input torque, as it was in the direction of increasing from zero, and a sudden fall from another "high" level A.sub.2 lower than the level A.sub.1 to the "low" state at another value of magnitude T.sub.3 (for clockwise direction) or T.sub.4 (for counterclockwise direction) of the input torque, as it was in the direction of decreasing to zero.
In this respect, the hysterestic width to be defined as the deviation between the rise and fall points T.sub.1, T.sub.3 (for clockwise direction) as well as that between the rise and fall points T.sub.2, T.sub.4 (for counterclockwise direction) was so small that, when the electric motor was turned "on" from "off" state thereof with rotation of the input shaft in either direction, auxiliary torque was excessively applied to the output shaft, thereby cancelling the phase delay that the output shaft had relative to the input shaft, thus reducing the level of the detection signal from the torque detection mechanism. Therefore, the armature current Am was then returned to the "low" level, which in turn gave rise to an enlarged phase difference between the input and output shafts, again turning "on" the electric motor. As a result, when the steering wheel was operated, in the region in which the torque acting on the input shaft was small, the electric motor alternately repeated "on" and "off", entering a hunting state.
The present invention has been achieved to effectively solve such problems in a conventional type of electromagnetic servo device, and particularly, of an electromagnetic servo device for electric type power steering systems for vehicles.